Microwave guide rotary joint



March 5, 1957 J. F. ZALESKI 2,784,383

MICROWAVE GUIDE ROTARY JOINT Filed March 17, 1954 I l Ll C 7 7 2 as f \3 Q a a 26 as as L 3\ a? s\ a I4 J as as L 2 7 5 1 B INVEN TOR.

JOHN F. ZALE$K| and the coaxial line. :pedance discontinuity partly due to the wide difference between the characteristic impedance of the rectangular guide and of the coaxial line.

United tates Patent MICROWAVE GUIDE ROTARY JOINT John F. Zaleski, Valhalla, N. Y., assignor to General Precision Laboratory Incorporated, a corporation of New York Application March 17, 1954, Serial No. 416,813

4 Claims. (Cl. 333-98) This invention relates to rotatable joints in microwave hollow guides and more specifically to joints which include transitions from rectangular guide to coaxial transmission line.

In many microwave applications it is desirable to provide relative rotary motion between two sections of waveguide transmission line. Under such circumstances it is usual to provide a joint consisting of a short coaxial line connecting two rectangular waveguide sections. Transitions are provided between each rectangular section At each transition there is an im- There is also a phase shift at each transition produced by reactive components. Both of these effects must be neutralized. The impedance transformation may be effected by a doorknob or other type of transition, appropriately dimensioned, and the reactance neutralization is ordinarliy effected by an iris or tuning screw in each rectangular guide or by a coaxial tuning stub at each transition. These irises and tuning screws, however, in any practical design can be constructed for perfect neutralization only when the energy flow is in a selected direction. When the energy flow is in the opposite direction some reflection will occur because of loss within the device.

The rotary joint of this invention is of this rectangular guide-coaxial line type, but the reactance neutralization tion of the outer conductor, or both kinds of sleeves may be employed together. In all cases the sleeve or sleeves are electrically centered between the two transitions. The diameter and length of the cylindrical enlargement or reduction are chosen with relation to the inner and outer coaxial diameters and to the dimensions of the transitions to effect complete neutralization of the phase shifts of both transitions. Since the combination is symmetrical it operates equally well for energy passage in either direction.

One purpose of this invention is to provide a rotatable joint in a microwave guide.

Another purpose of this invention is to provide a rotatable joint connecting two rectangular hollow guides and including a coaxial line section.

Still another purpose of this invention is to provide improved means in a microwave rotatable point for phase angle correction.

A further understanding of this invention may be secured from the detailed description and drawings, in which:

Figure 1 depicts a center cross section through a rotatable joint illustrating the principles of this invention.

Figure 2 indicates the use of a constricted outer con- FCC ductor to neutralize the geometrical discontinuity of a rotating joint.

Figure 3 illustrates a section of a rotating joint having the outer conductor restricted and the inner conductor enlarged.

Referring now to Fig. l, rectangular guides 11 and 12 are positioned parallel with their broad faces 13 and 14 facing each other. The broad face 13 contains a circular aperture indicated by its edges 16 and 16' and the broad face 14 contains a similar aperture indicated by its edges 17 and 17. The guides 11 and 12 are mechanically connected by an air-pressure-tight joint comprising a cylindrical re-entrant casing 18 fitted and soldered to guide 11 and a re-entrant flange member 19 fitted and soldered to guide 12, the s oldered joints .being indicated by the fillets 21 and 22 respectively. The flange member 19 -is retained in running fit relationto the casing 18 by retaining ring 23 and screws, such as, 24 and 24'. A flexible metal bellows 26 is soldered at one end 27 to the casing 18 and at its other end 28 to a ring 29 composed of bearing material. The surfaces of this ring and flange19 which meet at the surface 31 are precisely made so that the joint is airtight. The rotatable joint therefore is capable of being rotated yet, being airtight, can be em ployed in a pressurized microwave hollow guide system.

The cylindrical inner re-entrant portion 32 of the casing 18 is telescopically fitted with the inner re-entrant portion 33 of the flange 19 and these re-entrant portions taken together constitute the outerconductor of a coaxial line. The re-entrant portion 33 projects into a cylindrical recess in the re-entrant portion 32 to form a continuous inner surface broken only at the abutting end surfaces by the annular clearance 34 provided to permit free relative mechanical motion. The clearance space 34' is prevented from constituting an electrical interruption to the inner conducting coaxial surface by the use of a halfwavelength choke as is well understood in the art. The end 36 of the re-entrant portion 32 which projects past the re-entrant portion33 is made one-quarter wavelength long, terminating at edge 37. Likewise the space 38 .between the edge 37 and the bottom of the re-entrant. portion 33 is made one-quarter wavelength long so that microwave energy is transmitted across the clearance space 34 without interruption and leakage of energy is prevented. v j

The inner coaxial conductor includes a rod 39. This rod is coaxial with the cylindrical surfaces 32 and 33,

,and the rod surface comprises the coaxial line inner conducting surface The rod 39 is smoothly blended at its upper end into an enlargement 41 which is secured by a flange 42 and by a solder fillet 43 to guide 11. It is this enlargement which gives the transition its common fanciful name of doorknob transition. One purpose of enlargement 41 is to distort the microwave electromagnetic field within the waveguide so as to be coupled into the coaxial line and to produce a field therein and vice versa. Another purpose of enlargement 41 combined with that portion of rod 39 projecting into guide 11 is to serve as an impedance transformer between the coaxial line and the guide 11 to enhance the efficiency of energy coupling and to tend to eliminate reflection of energy back to the source of energy. However, while this construction serves as an impedance transformer, nevertheless a phase shift in the microwave energy remains, so for best matching of the joint at this point some auxiliary means for introducing either inductance or capacitance is required to neutralize phase shift. Without auxiliary means such a doorknob transition causes reflection of energy toward the source.

The other end of rod 39 is rotatably held in a hub member 44 which is soldered to a transition member 46 ing impedance.

which in turn is soldered to guide 12. Transition member 46 is similar in form to enlargement 41 and constitutes, with the part of rod 39 projecting into the guide, the lower transition. The cylindrical spaces 47 and 48 constitute a half-wave choke producing low impedance at the edge 49 to impinging microwave energy.

The rod 39 is provided with a conductive cylindrical enlargement S1 centrally positioned thereon. This enlargement terminates at an upper shoulder 52 and lower shoulder 53. its length L and diameter D are made such as to neutralize the phase shifts associated with both doorknob transitions. As a specific example of construction for use at 8800 me. p. s., the outer coaxial line diameter should be 0.436 inch, the inner coaxial line diameter 0.189 inch, the length L 0.485 inch and the diameter D 0.251 inch.

Although the cylindrical enlargement 51 is described as conductive, it as well as other matching devices described hereinafter may be made of dielectric material with suitably altered dimension-s.

' The cylindrical enlargement 51 being positioned electrically half way between the transitions, and the entire rotatable joint being substantially symmetrical about the center plane between the two transitions, equally good results are secured for energy passing through the rotata'oic joint in either direction.

The operation of phase-neutralizing component 51 may be described as follows. Energy applied through either rectangular guide, say guide 12, impinging on its associate transition is coupled into the coaxial line. The

other transition couples the energy into the output guide 11. Reflections of the impinging energy occur at the lower and upper transitions and at the two shoulders 53 and 52 of the cylindrical enlargement 51.

In addition, energy rellected from the upper transition tends to be re-reilected at the shoulders 52 and 53 and at the lower transition. These several reflections all combine to effect nearly perfect cancellation so that standing waves are substantially eliminated in either guide. Measured voltage standing wave ratios as low as 1.03 are attained in the input guide, with negligible reflection in the output guide.

in the operation of the enlargement 51 to neutralize phase shift as described, it is to be understood that the enlargement does not execute the function of transform- An impedance transformer necessarily has a length L which is a specific number of quarter wavelengths of the impinging energy, usually one or two. in the instant example, however, the length L is no commeasurable fraction or multiple or one-quarter wavelength.

If desired the enlargement or protuberance 51 may be .placed on the inside of the outer conductors, 32 and 33,

in which case the rod 39 is extended from end to end without enlargement. Such construction is illustrated in Fig. 2, which is a section of the coaxial portion of a rotating joint identical with that of Fig. 1, except that the cylindrical enlargement 51 is omitted and a cylindrical constriction 54 of the inner wall of the outer conductor is shown. This constriction is composed of a constricted section 54 of the re-entrant part 32 of re-entrant casing 18, and of a constricted section terminating a shoulder 54" of the re-entrant part 33 of the flange member 19. The two sections behave as a single uninterrupted cylindrical restriction of the outer surface because of the choke construction. The operation of the construction illustrated in Fig. 2 is similar to that of the construction of Fig. l.

The constructions of Figs. 1 and 2 can be combined as illustrated in Fig. 3, in which a small restriction 56 of the outer conductor is combined with a small enlargement 57 of the inner conductor to form jointly a geometrical or phase discontinuity matching device. This device behaves in operation as described in connection with the embodiment of Fig. 1.

Either or both of the hollow rectangular guides 11 and '12 may be replaced by hollow round or elliptical guide operated in the T511 mode.

What is claimed is:

l. A rotatable microwave joint comprising, two microwave rectangular hollow guides having their broad sides parallel, a coaxial microwave line joining said hollow guides, a transition for matching the resistive portions of the impedance values at each junction of said coaxial line with said hollow guides, an air-tight swivel joint in said coaxial microwave line, and a cylindrical section of said coaxial line positioned electrically equidistant from said transitions, said cylindrical section reducing distance between the inner and outer conductors, whereby the reactive portions of impedance discontinuities at the junctions of said coaxial line with said two hollow guides are nullified.

2. A joint in accordance with claim 1 in which said reduced distance in the cylindrical section is effected by enlargement of the inner conductor.

3. A joint in accordance with claim 1 in which said reduced distance in the cylindrical section is effected by reduction of the outer conductor.

4. A joint in accordance with claim 1 in which said reduction of the distance between inner and outer conductor is effected by both an increase in the inner conductor diameter and a decrease in the outer conductor diameter.

Microwave Transmission Circuits; vol. 9 of Radiation Laboratory Series; McGraw-Hill Book Co., Inc. copyright 1948; pages 446-455. Copy in Div. 69. 

